The present invention relates to the cooling of high-voltage batteries in vehicles that can be electrically driven. In particular, the present invention relates to improvements in the control of a refrigerant flow through the evaporator which is set up for cooling the high-voltage accumulator.
Specifically in electric vehicles and hybrid vehicles, lithium-ion batteries are very commonly used as energy stores. In this case, the thermal management of lithium-ion batteries is an important factor for the service life and the power output of the battery. For example, the spread of the temperature between the battery cells must not exceed a certain differential value, while at the same time, a maximum temperature of all of the cells must be adhered to. These requirements must be met even in the presence of large fluctuations in the outside temperature range. For example, for automobiles, a requirement may be that the lithium-ion batteries are available with full function in an outside temperature range from −10° to +40° C.
In the presence of cold temperatures below the freezing point, critical states may arise in a refrigeration circuit, which can lead to inadequate cooling or to an exceedance of the above limit values. Since, under winter conditions, there is always a very small pressure gradient in the refrigeration circuit, there is correspondingly a very small mass flow of cooling fluid. Given a constant refrigeration power of a cooler, intense overheating of the cooling fluid thus occurs. Said overheating however leads to a large spread in the temperature distribution within the battery cells, as the temperature of the cooling fluid differs considerably between the inlet and the outlet of the cooler. For example, intense overheating of the battery cells arranged furthest downstream in the cooling circuit may result.
It is therefore an object of the present invention to ensure reliable operation of a high-voltage accumulator that is cooled by way of an air-conditioning system, even in the presence of low ambient temperatures.
One embodiment of the invention proposes a method for optimizing the cooling of a high-voltage accumulator by way of an air-conditioning system in a transportation means. In this case, the transportation means may for example be a passenger motor vehicle, which in particular can be electrically driven. Other transportation means which can be driven by way of an electrochemical high-voltage accumulator are however likewise conceivable. Here, the method according to an embodiment of the invention encompasses, as a first step, and identification of an excessively low refrigerant flow through an evaporator for the high-voltage accumulator. In this case, an excessively low refrigerant flow leads (as described in the introduction) to a low dissipation of heat, and in particular to a large spread of the temperature distribution within the accumulator cells of the high-voltage accumulator. In response thereto, as a second step, there is proposed an in particular automatic reduction of heat losses within a condenser of the air-conditioning system in order to increase the refrigerant flow. Since it is the case in particular in the presence of low outside temperatures that the refrigerant flow drastically decreases owing to an excessively low temperature difference in the region of the condenser, it is possible in this way for the temperature difference to be increased, and for adequate cooling of the high-voltage accumulator by way of an evaporator which is thermally coupled to the high-voltage accumulator to be ensured. For a reduction in heat losses, use may be made, according to the invention, of various measures which are performed preferably without separate user interaction, that is to say automatically. In this case, in the context of the invention, a “heat loss” of the condenser is to be understood in particular to mean the heat loss balance within the condenser. In other words, according to the invention, an increased output of heat from the condenser can be compensated by way of an increased supply of heat. Here, in the context of the possible measures, possibilities are preferably favored which permit inexpensive and operationally reliable implementation in a series-production environment.
The subclaims relate to preferred refinements of the invention.
It is preferably the case that the excessively low refrigerant flow is identified in response to various findings. A first possibility consists in determining an exceedance of a predefined temperature within the high-voltage accumulator. For this purpose, various methods are conceivable which are basically known in the prior art. For example, a measurement of electrical characteristic variables of the high-voltage accumulator, an evaluation of a thermal element, an evaluation of an infrared sensor or the like are possible according to the invention. Estimating methods based on empirical values may also be used, taking into consideration an operating state of the transportation means and/or of the high-voltage accumulator. Alternatively or in addition, it is similarly possible for an exceedance of a predefined temperature spread between accumulator cells within the high-voltage accumulator to be determined. In other words, a temperature difference between a first accumulator cell and a second accumulator cell is determined and compared with a predefined threshold value. Alternatively or in addition, an undershooting of a predefined temperature difference between a refrigerant of the air-conditioning system and an ambient air flowing around the condenser may be determined. In this case, it is for example possible for the ambient air to be measured and compared with refrigerant temperatures within the air-conditioning system determined by measurement or empirically. In this case, on an empirical basis and in a manner dependent on a present operating state of the high-voltage accumulator, it is possible to predict whether a resultant refrigerant flow for ensuring an adequate dissipation of heat from the high-voltage accumulator will necessitate measures according to the invention. The above-stated measures provide advantageous findings which are easy to determine in terms of measurement technology, and in response to which a reaction according to the invention can advantageously be implemented.
It is preferably the case that, in order to reduce heat losses, at least a part of the refrigerant flow is conducted so as to bypass at least a part of the condenser of the air-conditioning system. In other words, a refrigerant flow is branched off at a location such that the branched-off part of the refrigerant flow does not flow through the entire length of the condenser. In particular, it is possible here for a part of the refrigerant flow to be conducted so as to bypass the entire condenser. This has the advantage that the condenser can be constructed without valves for branching off a part of the refrigerant flow, which reduces the component costs for the condenser. Alternatively, it is possible for the entire refrigerant flow to be conducted so as to bypass a part of the condenser. For this purpose, a valve arranged in the condenser may ensure that a bypass past the condenser can be of a short design which saves structural space.
An alternative or additional reduction of heat losses is realized according to the invention by way of a reduction of an ambient air flow through the compressor. In other words, the ambient fluid which dissipates the heat from the condenser is influenced such that the heat dissipation is reduced and thus the condenser temperature is increased. For example, the ambient air flowing to the condenser may be influenced by way of variable air-guiding means such that a reduced mass flow of ambient air flows to and/or through the condenser surface. If the condenser is arranged downstream of air-guiding means already provided for other purposes, it is thus possible according to the invention for a control signal for the actuation of the air-guiding means to be generated in response to an identification of an excessively low refrigerant flow through the evaporator for the high-voltage accumulator, in response to which control signal the throughput of ambient air through the condenser is reduced by way of the air-guiding means which are installed in any case. In this way, no additional hardware is required to realize the method according to the invention.
A further step according to the invention for reducing heat losses within the condenser consists in an additional infeed of heat into the condenser. This may be realized for example by way of a heating element which is operated for example using electrical or chemically stored energy (for example in the form of a fuel). In particular, the use of electrical energy makes it possible here to realize a simple and reliable design and inexpensive construction and operation. It is preferably also possible for waste heat generated in the transportation means to be used for heating the condenser. For example, for this purpose, a thermal coupling action between the condenser and a circuit for waste-heat dissipation may be temporarily improved. An example of a circuit for waste-heat dissipation is the cooling water circuit for dissipating the waste heat of a drive assembly of a transportation means. In this case, the thermal coupling action may simply be realized by virtue of the spacing between a section of the circuit and the condenser being reduced. Alternatively or in addition, an insulator situated between the circuit and the condenser may be temporarily removed or reduced. For example, it would be possible for a tank situated between the circuit and the condenser to be flooded with a heat-transporting fluid (for example water) in order to increase the thermal coupling action. Alternatively or in addition, it is possible for a cooling water flow through a heat exchanger, which is thermally coupled to the condenser, of the cooling water circuit of the transportation means to be increased. In other words, it would be possible for the cooling water flow through the heat exchanger to be increased by virtue, for example, of a bypass past the heat exchanger being shut off, and thus the entire cooling water flow duct that passes through the cooling water circuit then also flowing through the heat exchanger. According to the invention, the above-stated measures for the additional infeed of heat reduce the heat loss balance within the condenser and, in the presence of low ambient temperatures, lead to a more suitable temperature difference in relation to the condenser.
It is furthermore preferable for the air-conditioning system used according to the invention to additionally be used for influencing the climate in an occupant compartment of the transportation means. In other words, two parallel branches of the refrigerant circuit are used in order, on the one hand, to influence the interior compartment air or the temperature thereof and, on the other hand, to effect cooling according to the invention of the high-voltage accumulator of the transportation means. This permits an inexpensive implementation of the method according to the invention, and reduces the weight and structural space for the cooling of the high-voltage accumulator.
According to a further aspect of the present invention, there is proposed a control unit for a transportation means, in particular a passenger motor vehicle, for optimizing the cooling of a high-voltage accumulator by way of an air-conditioning system of the transportation means. In other words, the control unit is set up to carry out the method according to the invention as described above. For this purpose, said control unit receives input variables for the purposes of identifying an excessively low refrigerant flow, in particular through the evaporator for the high-voltage accumulator. Said input variables may for example be measurement results or empirical characteristic variables which permit present or predictive temperature determination. Furthermore, according to invention, the control unit is set up to generate output variables in order to react to the input variables in accordance with the invention. Output variables refer to signals which are suitable for reducing heat losses within a condenser of the air-conditioning system and thus for increasing the refrigerant flow. For example, this may involve an actuation of a valve for diverting a refrigerant flow so as to bypass the condenser. Alternatively or in addition, actuators may be actuated for the purposes of adjusting variable air-guiding means. Finally, the output variables are alternatively or additionally suitable for triggering an infeed of heat into the condenser. This may be realized for example by actuation of switching means which supply electrical energy to an electrical heating element on the condenser. Alternatively or in addition, the output variables may actuate actuators which improve a thermal coupling action between a cooling water circuit and the condenser. Furthermore, it is alternatively or additionally possible for the output variables to permit an increase of a cooling water flow through a heat exchanger which is thermally coupled to the condenser, for example by virtue of valves of a bypass of the heat exchanger being correspondingly actuated.
Further details, features and advantages of the invention will emerge from the following description and from the figures, in which:
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.